Studies are continuing to define the fimbrial adhesins of Actinomyces viscosus, an oral bacterium that colonizes tooth surfaces, forms dental plaque and contributes to the initiation of gingivitis and periodontal diseases. The type 1 fimbriae of actinomyces appear to be a principal adhesin for tooth surfaces as shown by studies of bacterial adsorption to saliva-treated hydroxyapatite. Recombinant DNA technology has been used to clone a structural gene that encodes the type 1 fimbrial subunit of A. viscosus strain T14V. This protein has a molecular weight of 62,000 daltons and is reactive with monospecific as well as monoclonal antibodies directed against actinomyces type 1 fimbriae. The type 2 fimbriae of actinomyces are antigenically and functionally distinct from type 1. Type 2 fimbriae account for the lactose-sensitive coaggregation of actinomyces with strains of Streptococcus sanguis and thus would be expected to promote interbacterial adherence within dental plaque. The gene encoding the type 2 fimbrial subunit of strain T14V has been cloned and the protein expressed in Eschedrichia coli has been partially purified. In addition to the study of type 2 fimbriae, immunochemical studies are continuing to define streptococcal carbohydrates that function as receptors for the actinomyces lectin. The receptor-associated polysaccharide of S. sanguis 34 has been purified and found to have N-acetylgalactosamine as the immunodominant group. These findings have begun to provide a structural basis for understanding the mechanisms of oral microbial adherence.